1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly, to such fuel cells that include passive water management techniques.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity from fuel and oxygen. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials in liquid form, such as methanol are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before the hydrogen is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external fuel processing. Many currently available fuel cells are reformer-based. However, because fuel processing is complex and generally requires costly components which occupy significant volume, reformer based systems are more suitable for comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as for somewhat larger scale applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is directly introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is the direct methanol fuel cell or DMFC system. In a DMFC system, a mixture comprised of predominantly methanol or methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidant. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate, as is discussed further hereinafter.
Typical DMFC systems include a fuel source or reservoir, fluid and effluent management systems, and air management systems, as well as the direct methanol fuel cell (“fuel cell”) itself. The fuel cell typically consists of a housing, hardware for current collection, fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system take place at and within the MEA. In the fuel oxidation process at the anode, the fuel typically reacts with water and the products are protons, electrons and carbon dioxide. Protons from hydrogen in the fuel and in water molecules involved in the anodic reaction migrate through the proton conducting membrane electrolyte (“PCM”), which is non-conductive to the electrons. The electrons travel through an external circuit which contains the load, and are united with the protons and oxygen molecules in the cathodic reaction. The electronic current through the load provides the electric power from the fuel cell.
A typical MEA includes an anode catalyst layer and a cathode catalyst layer sandwiching a centrally disposed PCM. One example of a commercially available PCM is NAFION® (NAFION® is a registered trademark of E.I. Dupont de Nemours and Company), a cation exchange membrane based on polyperfluorosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. A PCM that is optimal for fuel cell applications possesses a good protonic conductivity and is well-hydrated. On either face of the catalyst coated PCM, the MEA further typically includes a “diffusion layer”. The diffusion layer on the anode side is employed to evenly distribute the liquid or gaseous fuel over the catalyzed anode face of the PCM, while allowing the reaction products, typically gaseous carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen to the cathode face of the PCM, while minimizing or eliminating the accumulation of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM to the current collector.
Direct oxidation fuel cell systems for portable electronic devices ideally are as small as possible for a given electrical power and energy requirement. The power output is governed by the rates of the reactions that occur at the anode and the cathode of the fuel cell operated at a given cell voltage. More specifically, the anode process in direct methanol fuel cells, which use acid electrolyte membranes including polyperfluorosulfonic acid and other polymeric electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, water molecules are consumed to complete the oxidation of methanol to a final CO2 product in a six-electron process, according to the following electrochemical equation:CH3OH+H2OCO2+6H++6e−  (1)
Since water is a reactant in this anodic process at a molecular ratio of 1:1 (water:methanol), the supply of water, together with methanol to the anode at an appropriate weight (or volume) ratio is critical for sustaining this process in the cell. In fact, it has been known that the water:methanol molecular ratio in the anode of the DMFC has to significantly exceed the stoichiometric 1:1 ratio suggested by process (1), to guarantee complete anodic oxidation to CO2, rather than partial oxidation to either formic acid, or formaldehyde, 4e− and 2e− processes, respectively, described by equations (2) and (3) below:CH3OH+H2OHCOOH+4H++4e−  (2)CH3OHH2CO+2H++2e−  (3)
Equations (2) and (3) are partial anodic oxidation processes that are not desirable and which might occur if the ratio of water to methanol is not sufficient during a steady state operation of the cell. Particularly, as is indicated in process (3), which involves the partial oxidation of methanol, water is not required for this anode process and thus, this process may dominate when the water level in the anode drops below a certain point. The consequence of process (3) domination, is an effective drop in methanol energy content by about 66% compared with consumption of methanol by process (1), which results in a lower cell electrical energy output. In addition, it would lead to the generation of undesirable anode products such as formaldehyde.
Typically, it has been difficult to provide a desirable water/methanol mixture at the anode catalyst in a small, lower volume, compact DMFC technology platform. The conventional approaches to this problem can be divided into two categories:
(A) active systems based on feeding the cell anode with very diluted (2%) methanol solution, pumping excess amount of water at the cell cathode back to cell anode and dosing the re-circulation liquid with neat methanol stored in a reservoir; and
(B) passive systems requiring no pumping, utilizing reservoirs of methanol/water mixtures.
Class A systems, which are active systems that include pumping, can provide, in principle, maintenance of appropriate water level in the anode, but this is accomplished by dosing neat methanol from a fuel delivery cartridge into a recirculation loop. The loop also receives water, which is collected at the cathode and pumped back into the recirculating anode liquid. In this way, an optimized water/methanol anode mix can be maintained. The concentration is usually controlled using a methanol concentration sensor. The advantage of this approach is that a concentrated methanol solution comprised of a molecular fraction of at least 50% methanol, and preferably “neat” methanol (pure methanol) can be carried in the cartridge while a diluted methanol solution carried in the re-circulating loop supplies the required methanol to water ratio at the cell anode. Carrying a high concentration fuel source and recovering water from cell cathode reduces the amount of water needed to be carried in the cartridge and thus reduces the weight and volume of the reservoir and thus, the overall system. The disadvantage is that while neat methanol can be carried in the cartridge, the system suffers from excessive complexity due to the pumping and recirculation components as well as the concentration sensor, which can result in significant parasitic power losses and increases in the weight and volume of the power system. This can be particularly severe when the power system is used as a small scale power source.
The class B systems, comprising passive systems, have the advantage of system simplicity achieved by eliminating water recovering, pumping and recirculating devices by using a design that carries a mixture of water and methanol in the fuel reservoir. This type of system can be substantially, or even completely passive, as long as the rate of water loss through the cathode is adjusted by the water carried “on board” the fuel cell system, typically within the fuel reservoir. The problem with this approach is that it requires that a significant amount of water which has no intrinsic energy content, be carried in the fuel reservoir or cartridge.
A fuel cell system that adapts the best features of both the Class A and Class B systems, without the disadvantages of these two known systems, would be most advantageous for portable power applications. However, attempts to develop such a system heretofore have been unsuccessful due to the challenges associated with the fundamental feature of process (1), i.e., the need to provide water to the anode at, at least, a mole fraction of 50%, or at 25% by weight of methanol in the mixture with methanol. In fact, in the common approach, a solution of as high as 98% water (and 2% methanol) has to be introduced to the anode aspect of the membrane electrolyte assembly, in order to minimize the amount of methanol that passes through the membrane electrolyte without participating in the anodic reaction (a phenomenon known as methanol “cross-over”).
Consequently, the possibility of supply of highly concentrated methanol, including 100% methanol, directly from a reservoir into the anode compartment, has not been considered practical to date without, at the same time, actively supplying water to the anode catalyst by concurrently collecting water from the cathode and pumping it back to the anode of the fuel cell. In other words, the introduction of neat methanol or highly concentrated methanol solution (>10% by weight) directly to the anode in a passive system has not, up to now, been considered feasible, due to the expected results of significant loss of methanol flowing across the membrane and anode processes other than process (1), noted above.
In such cells the water available is exclusively generated internally at the cathode, and therefore water distribution throughout the cell must be managed carefully. In considering the management of water, there are several competing considerations to be taken into account. The fundamental challenge is to generate a sufficient flow of cathodically generated water, from the cathode to the anode to provide for the complete oxidation of methanol as per process (1). To do so requires that a portion of the cathodically generated water be pushed back to the anode and have any excess water released as water vapor from the cathode aspect of the fuel cell. In turn, this means that a balance between passive, evaporative loss of water from the cathode and the confinement and controlled distribution of water within the cell must be achieved. There remains a need therefore for a direct oxidation fuel cell system which includes techniques for passively delivering an effective supply of liquid water back to the anode while preventing cell dry out due to excessive water vapor loss and without significantly impeding air access to the cathode catalyst layer as result of excess liquid water buildup in the cathode.
It is thus an object of the invention to provide a direct oxidation fuel cell system that includes passive water management techniques which prevent both cell dehydration and cathode flooding.
It is a further object of the present invention to provide a direct oxidation fuel cell system that is capable of operating on neat methanol or a highly concentrated methanol solution as a fuel source and delivering this fuel directly to the anode, but also allows sufficient water to be present at the anode to result in the efficient completion of anode process (1) at the membrane electrolyte using passive water management techniques, i.e., without recovering water from the cell cathode and without using recirculation and pumping mechanisms. More specifically, it is a further object of the invention, to develop direct oxidation fuel cell applications where air is supplied to the cathode without forced air flow, where water is managed passively, and where a concentrated methanol solution is supplied to the anode chamber of the fuel cell.